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Sick of having to replace the spark plug on your dirt bike almost every ride? Chances are, it’s a fairly easy fix. But there can be several variables to the equation. Incorrect jetting is the number one cause of plug fouling, but we’ll get a little deeper into what the most common causes could be and how you can fix them before spending a ton of cash on spark plugs.

No Air = No Combustion

Dirty Air Filter

First thing to do if your spark plug has been fouled is to check the air filter. If it’s really dirty, that makes it difficult for air to get through to the engine, causing a rich condition. Either clean it thoroughly with some Air Filter Cleaner, or buy a new filter. Before you put it back in the air-box, treat with some air filter oil or spray. BE CAREFUL, though, because adding too much oil can also clog up the filter, resulting in another fouled plug.

What Mixture Are You Smoking?

(For 2-Strokes) While the gas/oil ratio doesn’t equate to what the jetting is, you can still foul plugs, or worse, if the mixture is wrong. Depending on your bike, it’s best to look at your owner’s manual for the correct ratio. For most 2-stroke motocross bikes, a ratio is 32:1 or 40:1, gas being 32 or 40, and oil being 1. For those that don’t it, it means that for every 128 ounces (one gallon) of gas, you add 4 ounces of 2-stroke oil for a 32:1 ration, or just over 3 ounces for a 40:1 ratio.

If you have too much oil in the mixture (such as 12:1), then you will probably end up fouling the plug because it is more difficult to burn that much oil. Don’t try to cut corners with the oil though, because it’s very important for your dirt bike’s engine. 2-stokes need the oil for lubrication on the cylinder walls, so if you don’t have enough (or any for that matter) then the cylinder walls will run dry, causing it to overheat and seize the engine very quickly. So it’s very important that you put in the right mixture pre-mix to your 2-stroke gas tank.

Also, 2-strokes have reeds that air and fuel go through into the cylinder, so if they are cracked or broken the bike will not run right. This could cause the plug to foul, so before you go out and buy anything expensive, make sure the reeds are still good.

Too Rich

Turn Up The Heat!

While this usually isn’t the main problem, it could be that your spark plug isn’t burning hot enough. Having a hotter spark will ignite the fuel stronger, leaving less residual gas/oil in the cylinder that could cause the plug to foul. This isn’t the best choice, but I’d say it’s safe to go one plug hotter than stock. Spark plug manufacturers aren’t always the same, so make sure you find out how they rate them.

4-Strokes Foul Plugs Too…

Don’t be fooled into thinking that only 2-strokes foul plugs. 4-Strokes, although it is not as common, do foul plugs. Like I mentioned before, improper jetting is the main reason why dirt bikes foul spark plugs. Almost every motocross bike comes rich from the factory (especially Honda 2-Strokes when they were being made). It’s usually a pretty simple fix, but many riders are too lazy and would rather continue buying plugs instead of spending a few bucks a jets.

Can 4-Strokes Foul Plugs?

If you know for sure it’s a jetting issue that’s causing the fouling (you’ll know this by looking at the plug, which I’ll cover on how to read one in a future article), then the first thing you should do is go down a size or too on the main jet. Your goal is to get a crisp throttle response at any rpm. If you ride in the lower rpm range often, then you’ll want to go down on the pilot jet and lean out the clip position (learn the Basics of Jetting Here). By leaning out your bike, your bike will not foul plugs as easily, allowing you to ride all day long.

You Can Always Just Ride Harder

If you want to go the easy route (well not really!), just ride the bike harder. Spark plugs usually foul at low RPM’s, so if you’re wringing your bike out you shouldn’t have as much of a problem!

IF NOTHING ELSE WORKS!!!

If you’ve tried all of the above, then there’s a chance your dirt bike will need an engine rebuild. A worn out piston can cause a rich condition, which results in the plug fouling. So if the top-end hasn’t been rebuilt for a while, you’ll want to do that. Not only can this solve your plug/jetting issues, but it may prevent a major engine failure in the near future because of worn out parts.

Good luck, and keep that bike running strong!

-Tom Stark

Wikipedia: Revolutions per minute is a measure of the frequency of a rotation. →